Method for treatment of suspensions in movement

ABSTRACT

An apparatus and method are provided for treating flowing suspensions, extracting liquid from the suspensions and, if desired, introducing a treatment liquid into the flowing suspensions. Suspension flows in an elongated container from an inlet to an outlet in a first direction, and a screen surface is mounted within the container for screening a portion of the liquid from the suspension. The screen body is elongated in the direction of elongation of the container and defines a screen cavity volume within it, the volume tapering inwardly in the first direction. A baffle is disposed within the screen cavity volume. During movement of the screen in a second direction, opposite the first direction, at a higher speed, backflushing of the screen takes place, substantially evenly over the whole of the screen surface. Displacement liquid is introduced into the container, at at least one level along the inward taper of the screen body, so that during liquid extraction a crosswise displacement with treatment liquid is obtained through substantially all suspension flowing through the container. The container preferably has a vertical orientation, the inlet disposed above the outlet.

This is a division of application Ser. No. 223,741 filed Jan. 9, 1981,now U.S. Pat. No. 4,368,628.

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to an apparatus and method for treating a flowingsuspension. Liquid is continuously extracted from the flowingsuspension; if the extraction is combined with the introduction of atreatment liquid, then treatment of the suspension occurs, while if theextraction is not accompanied by an introduction of treatment liquid,dewatering of the suspension occurs.

When used for dewatering, the apparatus according to the presentinvention is of the same general type as the dewatering apparatus shownin U.S. Pat. No. 4,041,560 (the disclosure of which is herebyincorporated by reference herein) wherein a screen body is reciprocatedin a direction generally parallel to a suspension flow, withbackflushing of the screen openings taking place during movement of thescreen opposite to the direction of movement of the suspension. Whileprior art systems such as disclosed in U.S. Pat. No. 4,041,560 aregenerally successful in accomplishing their desired result, there areseveral areas where improvement is possible. The apparatus according tothe present invention is advantageous compared to such prior artassemblies in that the backflushing takes place substantially evenlyover the whole of the screen surface. The apparatus and method accordingto the present invention are also advantageous over prior art systems inthat they facilitate the introduction of a treatment liquid into theflowing suspension, the treatment liquid introduction being practiced sothat during liquid extraction with the screen body a crosswise flow oftreatment liquid is obtained through substantially all suspensionflowing through the container. This makes the apparatus and methodaccording to the present invention extremely versatile for performing avariety of treatment functions.

The apparatus and method according to the present invention may be usedfor dewatering or treating a wide variety of suspensions, but areparticularly useful for dewatering and treating comminuted cellulosicfiber material suspended in liquid, which liquid can either be water ora chemical treatment liquid, such suspensions commonly being used andproduced during the production of paper products.

According to one aspect of the present invention, apparatus forcontinually extracting liquid from a flowing suspension is provided. Theapparatus includes an elongated container through which suspension to betreated flows in a first direction, an inlet and an outlet beingprovided adjacent opposite ends of the container in a dimension ofelongation thereof. Screen means are provided for removal of a portionof the liquid from the suspension flowing through the container, andextending generally in the direction of elongation of the container. Thescreen means include a perforated screen surface defining a screencavity volume therewithin. An outlet from the screen cavity volume isconnected to an area remote from the container, and bearing means mountthe screen means for movement generally in the dimension of elongationof the container, and means are provided for moving the screen means inthe dimension of elongation of the container, both in the firstdirection and in a second direction opposite the first direction. Bafflemeans are disposed within the screen cavity volume for cooperating withthe screen surface so that backflushing of the screen surface takesplace during movement thereof in the second direction, the backflusingbeing substantially even over the whole of the screen surface.

According to another aspect of the apparatus according to the presentinvention, the screen means includes a screen surface defining a screencavity volume therewithin, the cavity volume tapering inwardly in thefirst direction so that it has a larger cross-sectional area at thepoints closer to the container inlet than at points closer to thecontainer outlet. According to both described aspects of the presentinvention, preferably the screen surface tapers inwardly so that it isspaced farther from the container interior sidewalls at points closer tothe container outlet than it is at points closer to the container inlet.

The container preferably is substantially circular in cross-section,having a substantially constant diameter along the dimension ofelongation thereof, and treatment liquid introduction is provided byannular baffle plates disposed at at least one position along the inwardtaper of the screen body at the container interior sidewall. The screenmoving means preferably moves the screen means at a first speedcomparable to the speed of flow of suspension in the first direction,and at a second speed, much greater than the first speed, in the seconddirection. Withdrawal of suspension from the container outlet may befacilitated by a rotating scraper.

According to an exemplary method of the present invention, treatment ofthe suspension flowing through a container in a first direction isprovided. The method comprises the following steps: Introducingsuspension under a motive force into the container inlet and withdrawingit from the container outlet so that it flows in the first directionwithin the container. Effecting movement of the screen body in the firstand second directions within the container to extract some of the liquidfrom the moving suspension within the container so that it flows intothe volume defined by the screen body. Withdrawing liquid passing intothe volume defined by the screen body from that volume to an area remotefrom the container; and introducing treatment liquid into the container,from an interior sidewall of the container, at at least one positionalong the inward taper of the screen body, so that during liquidextraction with the screen body a crosswise flow of treatment liquid isobtained through substantially all suspension flowing through thecontainer.

The introduction of treatment liquid may be practiced utilizing anannular baffle plate provided at an interior sidewall of the container,the treatment liquid being introduced so that the liquid initially has avelocity component in the first direction and is introducedsubstantially evenly around the circumference of the container.Alternatively, or additionally, treatment liquid introduction pipes maybe disposed at various points within the container, this beingespecially useful where a plurality of screen surfaces are provided, orthe suspension flows both centrally and exteriorly of an annular screensurface.

It is the primary object of the present invention to provide anapparatus for continuously extracting liquid from a flowing suspensionwith substantially even backflushing of a screen surface thereof, andfor providing the effective introduction of treatment liquid into aflowing suspension in such a device. This and other objects of theinvention will become clear from an inspection of the detaileddescription of the invention, and from the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 through 5 are longitudinal cross-sectional views of variousembodiments of exemplary apparatus according to the present invention,portions of the embodiments in FIGS. 2 and 3 being cut away for clarity;

FIG. 6 is a top plan view of another embodiment of the apparatusaccording to the present invention;

FIG. 7 is an perspective cross-sectional view of the embodiment of FIG.6; and

FIG. 8 is a side view, partly in cross-section and partly in elevation,of yet another embodiment of the apparatus according to the presentinvention.

DETAILED DESCRIPTION OF THE DRAWINGS

The drawings illustrate a wide variety of configurations that theapparatus according to the present invention may take, each of which iscapable of practicing a method of treating a flowing suspension byextracting liquid therefrom and introducing treatment liquid thereinto.In each embodiment, comparable structures are referred to by comparablereference numerals, except that the numeral is preceded by a differentnumber for each embodiment. For instance, in the embodiment of FIG. 1,the container through which suspension to be treated flows is indicatedby reference numeral 10, while in the FIG. 2 embodiment it isillustrated by reference numeral 110, in the FIG. 3 embodiment byreference numeral 210, etc. The general concept of the present inventionwill now be described with respect to FIG. 1, and then the particularcharacteristics of each further embodiment will be described.

The apparatus according to the present invention includes an elongatedcontainer 10, which, as illustrated in FIG. 1, preferably is circular incross-section and has a substantially constant diameter in the dimensionof elongation AB thereof. An inlet 12 is provided adjacent one end ofthe container 10, and an outlet 14 adjacent the other end. Suspensionflows through inlet 12 in the direction of the arrows, this being afirst direction A, and then out the outlet 14. In the FIG. 1 embodiment,sealing covers 15, 16 are provided at the opposite ends of the container10, and the inlet and outlet 12, 14, are substantially perpendicular tothe dimension of elongation AB of the container 10. The suspension to betreated by the apparatus may comprise comminuted cellulosic fibrousmaterial, or the like.

The apparatus illustrated in FIG. 1 further comprises screen means,shown generally at 18, for removal of a portion of the liquid from thesuspension flowing through the container 10. The screen means 18 extendsgenerally in the dimension of elongation AB of the container 10, andincludes a perforated screen surface 19 which defines a screen cavityvolume 20 therewithin. An outlet 22 extends from the screen cavityvolume 20 to an area remote from the container, such as reservoir 23.Liquid extracted from the flowing suspension flows, as illustrated bythe arrows in FIG. 1, through the screen surface 19 and then flowsthrough the outlet 22, which in the FIG. 1 embodiment extends throughthe end wall 15.

Bearing means, such as the bearing body 25 and the annular sealingbearing component 26, mount the screen surface 19 for movement indirection A, and in direction B, opposite direction A. As illustrated,the movement is desirably linear. The apparatus further comprises meansfor moving the screen means 18 in the dimension AB in the first andsecond directions A, B, respectively. Such moving means preferablycomprises a fluid cylinder (pneumatic or hydraulic) 28 or like drivedevice. The drive device 28 is operatively connected to the screensurface 19 by any desirable means. In the FIG. 1 embodiment, connectionbetween the screen surface 19 and the drive device 28 is via a shaft 29which passes through seal 30 in container end wall 15, and a paraboloidof revolution body 31, which is welded at opposite ends thereof to shaft29 and screen surface 19.

In the FIG. 1 embodiment, the paraboloid of revolution body 31 comprisesbaffle means disposed within the screen cavity volume 20 for cooperatingwith the screen surface 19 so that backflushing of the screen surface 19takes place during movement thereof in the second direction B. Theparticular design and operative relationships between the baffle means31 and screen surface 19 are such that uniform backflushing takes placesubstantially evenly over the whole of the screen surface 19, ratherthan--as in some prior art systems--being more concentrated at the endsof the screen surface remote from the extraction liquid outlet.

The screen surface 19 tapers inwardly in the direction A so that it hasa larger cross-sectional area at points thereof closer to containerinlet 12 than at points closer to container outlet 14. Since thecontainer 10 preferably is of substantially constant cross-sectionalarea throughout the length thereof, the screen surface 19 will thus bespaced farther from the container interior sidewall(s) 33 at pointscloser to the container outlet 14 than at points closer to the containerinlet 12.

When the apparatus illustrated in FIG. 1 is operated to effectsuspension dewatering, preferably the drive device 28 moves the screenmeans 18 in first direction A at substantially the same speed as thesuspension flow, and then moves it at a much greater speed in seconddirection B. During movement of the screen means 18 in direction A, adifferential pressure is provided causing liquid to pass through thescreen surface 19 into the cavity volume 20, ultimately to pass throughextraction liquid outlet 22 to reservoir 23. During movement of thescreen means 18 in direction B backflushing takes place, the liquidbeing expelled from the cavity volume 20 to flush off suspensionparticles that may adhere to the screen surface 19, the backflushingbeing substantially even over the whole of the screen surface 19.

The apparatus of FIG. 1 also is readily utilizable for treatment of theflowing suspension, in addition to being utilizable for dewateringthereof. Treatment liquid introduction means, illustrated generally at35, are provided for introducing treatment liquid (such as a pulpbleaching liquid, wash liquid, etc.) into the container 10. One, two, orany other number of treatment liquid introduction structures 35 may beprovided as required for a particular operation. As illustrated in FIG.1, the treatment devices 35 are positioned along the container 10 alongthe inward taper of the screen body 19 so that during liquid extractionwith the screen means 18 a crosswise flow of treatment liquid isobtained through substantially all suspension flowing through thecontainer. In the embodiment illustrated in FIG. 1, the treatment liquidintroduction means 35 comprises, for each location, a baffle plate 37spaced a slight distance from the container interior sidewall 33 by anupper annular wall 38, and open adjacent the end thereof most remotefrom the container inlet 12. Inlet conduit 35 introduces the treatmentliquid into the volume between the baffle plate 37 and the interiorsidewall 33, the liquid flowing around the entire circumference of thesidewall 33, and, because of the shape of the baffle 37, initiallyhaving a velocity component in the direction A when contacting thesuspension flowing through the container 10. Such an arrangement,combined with the even extraction and backflushing of the screen surface19, effects a uniform treatment of the suspension while flowing frominlet 12 to outlet 14.

In order to facilitate discharge of the suspension through the outlet14, a rotating scraper, illustrated generally at 40, may be provided.The scraper includes drive device 41, rotatable shaft 42 mounted throughseal 43 in end wall 16, and scraper blades 44.

The apparatus of FIG. 1 may be operated in a number of orientations,depending upon particular end-use. However, it is especiallyadvantageous to arrange the container 10 so that the dimension ofelongation AB thereof is vertical, with the inlet 12 being disposedabove the outlet 14. In this way, the best conditions for evensuspension flow and treatment are obtained since the friction force dueto contact between the screen surface 19 and the suspension, and theweight of the moving parts, act in the direction of gravity as thescreen surface 19 is being moved in direction A. Therefore, very littleforce and power is necessary. When moved in the opposite direction B, arelatively high power input is required to initiate the movement, butafter a short distance the back-flushing through the screen holesdecreases the friction between the screen surface 19 and the suspensionso that relative movement therebetween is facilitated. Gravity then actsadvantageously in terminating movement of the screen means 18 indimension B, the movement termination being smooth and without anysignificant shock.

The screen surface 19 may be constructed from a wide variety ofmaterials. For instance, it may comprise a rough-perforated sub-skirtwhich is coated with woven steel cloth, nylon cloth, or the like. Thesurface 19 may be conical or wedge-shaped with a smooth taper, or it maybe formed in a stair-step arrangement, being formed by cylinders ofsmaller and smaller diameters, with screen slots formed at the jointsbetween the cylinders so that the screen slots are located in the shadowof the suspension movement over the screen "stairs". The screen openingsor slots may be circular, elongated, or any other configurationdepending upon the particular design and the suspension to be treated.

In the embodiment illustrated in FIG. 2, the suspension inlet 112 isdisposed at a raised portion of the container 110, and at the inlet arotating scraper, illustrated generally at 150, is provided. The scraper150, like the scraper 40 illustrated in FIG. 1, includes a conventionaldrive 151, rotatable shaft 152 passing through seal 153, and scraperblades 154, and provides for even introduction of the suspension intothe container 110. Also in this embodiment, a plurality of linear drives128 are provided for effecting movement in dimension AB of the screenbody 119, the reciprocating shafts 129 being directly connected to thescreen body 119, and the interior paraboloid of revolution body 131being connected at the bottom end thereof to the screen surface 119, butnot being connected at the top thereof.

In the embodiment illustrated in FIG. 3, the baffle means 231 has adifferent design. In this design, the paraboloid of revolution interiorbody has an open end 256 adjacent the point of connection of thereciprocating rod 229, the rod 229 being connected to the baffle body byspider arms 257. In this embodiment, since the open end 256 is providedthe baffle body 231 will fill with extraction liquid, the movement ofthe screen surface 219 thereby also resulting in movement of the liquidenclosed within the baffle body 231, the total mass to be acceleratedthus being larger. With this arrangement, however, the design of thebaffle means 231 can be somewhat simpler, particularly for the treatmentof suspensions under high pressure.

In the FIG. 4 embodiment, the suspension outlet 314 is formed as the endof the cylindrical container 310. The reciprocating rod 329 is connecteddirectly at end 360 thereof to the screen surface 319. However, in thisembodiment a stationary baffle means 331 is provided having a conicalexterior corresponding generally in taper to the taper of the screensurface 319. Thus, during fast upstroke of the screen 318, the spacingbetween screen surface 319 and the stationary baffle means 331 will beinstantly reduced. This creates a small relative pressure increase ofthe filtrate liquor trapped between the two surfaces. Part of thatfiltrate must therefore pass (return) through the screen surfaceperforations and form a liquor film between the outside of the screensurface and the pulp suspension.

The combined effect of the conical shape of the screen means and thestationary baffle conforming in shape with same ensures absolutelyuniform back-flush effect and thus creates a liquor film of uniformthickness all over the screen surface on the pulp suspension side of thescreen during the fast (return) upstroke.

Slide bearing means 361 is provided at the end of the conical bafflemeans 331 to allow the reciprocating rod 329 to move with respectthereto. Extraction liquid flows from the screen surface 319 to theextraction liquid 322 through holes 362.

In the embodiment of FIG. 5, the suspension inlet, drive devices 428,etc., are identical to those illustrated in the FIG. 2 embodiment. Thepurpose of the stationary baffle means 431 is identical to that of FIG.4. In the FIG. 5 embodiment, the stationary baffle means thus conformsin shape and conicity with the screen surface 419. However, in thisembodiment, the cylindrical prolongation 466 of the baffle meansprotrudes the screen means at the bottom of the screen surface 419. Inthis embodiment, also, slide bearings 465 are provided at the bottom endof the screen surface 419, cooperating with lower cylindrical extension466 of baffle means 431, the cylindrical extension 466 being closed offby lid 467. If desired, a partition wall, shown in dotted line at 468 inFIG. 5, may divide the interior of the baffle means 431 into twocavities. The liquid extracted below the partition wall 468 can beremoved by a separate structure (such as a pipeline penetrating thecontainer 410 top cover, such pipeline not illustrated in FIG. 5). Oneform such a withdrawal could take would be an interior pipe extendingwithin the conical baffle means 431 from the chamber below partition 468up to hollow supporting arm 469, connected to another extraction liquidoutlet (not shown). By providing the interior partition 468, onetreatment liquid (e.g., bleaching chemical) can be introduced throughthe top treatment liquid introduction structure 435, while anothertreatment liquid (e.g., a wash liquid) may be introduced through thebottom treatment liquid inlet 435, with separate extractions.

Of course, the baffle means 431 may be divided into a plurality of otherchambers, if desired, and each such chamber may cooperate with adifferent treatment liquid introduction structure 435. In this way, thesuspension may be treated countercurrently, with the extracted liquidfrom each stage being used as the treatment liquid for the previousstage, with clean wash water being added to the suspension through thebottommost (closest to the outlet 414) treatment liquid introductionstructure 435.

In the FIGS. 6 and 7 embodiment, the container 519 is shown as astructure that is quadrate in cross-section, having an open top 512serving as the suspension inlet, and an open bottom 514 serving as thesuspension outlet. In this embodiment, a plurality of screen means(e.g., three) 518 are provided, which each may have a separate drivedevice 528, or may be commonly actuated where desired. Eachreciprocating rod 529 may be connected to a screen surface 519 by spiderarms 557, which pass through slots 570 (which also provide forextraction liquid passage) formed in the interior baffle means 531. Thebearing means 530 for the reciprocating rods 529 are formed in an upperclosed wall 573 of baffle means 531. In this embodiment, both the screensurfaces 519 and baffle means 531 are truly wedge-shaped.

In the FIGS. 6 and 7 embodiment, the treatment liquid introductionstructures 535 are substasntially different than in the otherillustrated embodiments. The treatment liquid introduction structurestake the form of a plurality of pipes 580, 581, disposed adjacent thesidewalls 533 of container 510, each pipe having a series of openingsformed along the length thereof, and interiorly mounted fluiddistribution pipes 582, 583 also may be provided. The interior pipes582, 583 may be innerconnected by dividing wall portions 584, whichserve to divide the flow more evenly between the screen bodies 518. Thepipes 580-583 may have any desired configuration in order to provide aminimum of resistance to the suspension flow, while still providing evenintroduction of treatment liquid. The treatment liquid is actuallyintroduced into the pipes 580 through 583 through the conduits 539 (seeFIG. 6).

In the embodiment illustrated in FIG. 8, instead of the screen surface619 being defined as a right circular cone (or truncated cone), it isdefined as a generally toroidal structure, again tapering inwardly inthe first direction A. This provides the screen means 618 with anannular cross-section so that suspension flows both inwardly of andexteriorly of the screen surfaces 619. Thus, in this embodiment, thetreatment liquid is introduced, in addition to underneath the baffles637, through the interior liquid introduction structure 690. Theinterior fluid introduction structure 690 comprises a tubular shaft 691which communicates with, and is rotatable with, the shaft 642 for thescraper blades 644, driven by drive 641. Treatment liquid is introducedthrough conduit 692 to the interior of the pipe 691, and then flowsthrough openings 693 in the top of the conduit 691, between the volumedefined by the hat-shaped baffle body 694, out the open bottom 695 ofthe baffle body 694. In this way, the treatment liquid is alsointroduced evenly into that portion of the suspension flowing centrallyof the screen means 618.

In the FIG. 8 embodiment, it will be seen that the extraction liquidwithdrawal conduit 622 penetrates the sidewalls 633 of the container610, rather than extending through the top wall thereof. This connectionalso provides for mounting of the baffle means 631, an extension ofwhich forms the bearing mens 625. While one annular screen body has beenillustrated in FIG. 8, it will be readily apparent that a plurality ofconcentric annular screen bodies may be provided within the container610, with a suitable corresponding number of treatment liquidintroduction structures (690 or the like) if suspension treatment ispracticed. If a plurality of such annular screen bodies are provided,the distribution thereof should be such that the distance between thetreatment liquid supply devices and the closest screen body will beabout 0.1 to 0.5 meters in order to obtain the best possible conditionsfor displacement. About the same condition is generally valid withrespect to the thickness of the suspension layer if liquid is not added,and the apparatus is used for thickening only.

Each embodiment according to the present invention will be useful fordiffering operating and suspension conditions, etc. For instance, theembodiment of FIG. 5 is particularly useful where very little stirringof the suspension is desired, especially if the bearing seals 426, 465are made with as small a diameter as possible relative to the diameterof the bearing bodies 425, 466. For the embodiments wherein thecontainer (e.g. 10, 610) is circular in cross-section, super-atmosphericpressure conditions may be tolerated very well, and suitable adjustablecontrol devices may be utilized in conjunction with the inlets andoutlets maintaining continuous flows thus controlling the dewateringand/or displacement (wash) effects. Furthermore, the screen backflushingconditions may be controlled by a suitable number and dimensioning ofopenings within the baffle bodies (e.g., openings 462 in the FIG. 5embodiment) by providing the baffle means with a parabolic or conicalshape, as in the FIGS. 1 through 3 embodiments, or by having astationary baffle means with substantially the same conicity as thescreen surface, as in the FIGS. 4 through 8 embodiments. It is possibleaccording to the present invention to provide substantially evenbackflushing of the screen surface, minimizing the tendency for liquidto collect in the rear of the screen body and thereby clean the rearpart to a larger extent than in the front part. Where more than onescreen body is provided, the screen bodies may be moved synchronously,or separately, depending upon the particular situation.

An exemplary manner of operation of an apparatus according to thepresent invention will now be described, with particular reference toFIG. 5, the suspension being sulphate pulp withdrawn from a KamyrContinuous Digester. For such a structure, the screen body may have amaximum diameter of 750 millimeters, a perforated length of 2.75 meters,with an active screen area of 6 square meters. The inlet and outletpressure is maintained at 5 atmospheres, with the inlet and outlet pulpconcentration being about 9.5% (pulp concentrations of about 8 to 15%being most effectively treated). The inlet pulp temperature is 80° to85° C., the wash liquid temperature 105° to 110° C., the thickness ofthe pulp bed about 140 millimeters, and the cone angle about 2°.

The suspension is introduced through inlet 412, and passes as indicatedby the arrows in first direction A, moving toward outlet 414. As itflows in the container 410, the cylinder 428 moves the screen surface419 up and down, sliding on bearing means 425, 426 and 466, 465. Duringmovement in direction A, it moves at substantially the same velocity asthe suspension being treated, while in direction B it moves at asignificantly greater speed. The length of the stroke in the directionAB is preferably about 20 to 50 centimeters (or a maximum of about 70centimeters).

As the suspension flows from inlet 412 to outlet 414, liquid therefromis extracted through the screen surface 419, and flows through openings462 ultimately to extraction liquid outlet 422 and to reservoir 423.Treatment or displacement liquid, such as washing liquid for sulphatepulp, is introduced at introduction structures 435, being dispersedevenly around the circumference of the container 410, and initiallyhaving a downward velocity component, being identical to that of thepulp suspension, effecting crosswise flow of treatment liquid throughsubstantially all the suspension flowing through the container 410.

The washed sulphate pulp withdrawn through the outlet 414 when themethod as recited above is practiced is eminently suitable, and isobtained while maintaining a much higher screen loading (while stillmaintaining a safe function) compared to prior art systems.

It will thus be seen that according to the present invention an improvedmethod and apparatus have been provided for the extraction of liquidfrom a flowing suspension and/or the treatment of the suspension with atreatment liquid.

While the invention has been herein shown and described in what ispresently conceived to be the most practical and preferred embodimentthereof, it will be apparent to those of ordinary skill in the art thatmany modifications may be made thereof within the scope of theinvention, which scope is to be accorded the broadest interpretation ofthe appended claims so as to encompass all equivalent structures andmethods.

What is claimed is:
 1. A method of treating a suspension flowing througha container in a first direction, the container elongated in a dimensioncoextensive with said first direction and having mounted therein ascreen body reciprocal in the dimension of elongation of the containerboth in the first direction and in a second direction opposite the firstdirection, the screen body having a perforated screen surface defining ascreen cavity volume therewithin, said method comprising the stepsof:(a) introducing suspension under a motive force into the containerinlet and withdrawing it from the container outlet so that it flows inthe first direction within the container; (b) effecting movement of thescreen body in the first and second directions within the container toextract some of the liquid from the moving suspension within thecontainer so that it flows into the screen cavity volume; (c)withdrawing the liquid passing into the screen cavity volume from thatvolume to an area remote from the container; and (d) introducingtreatment liquid into the container, from an interior sidewall of thecontainer, so that during liquid extraction with the screen body acrosswise flow of treatment liquid is obtained through substantially allsuspension flowing through the container; and wherein at the points oftreatment liquid introduction a baffle plate is provided directing thetreatment liquid in the first direction, and substantially evenly aroundthe entire circumference of the container, and wherein step (d) ispracticed by introduced treatment liquid into the container adjacenteach baffle plate so that the treatment liquid initially has a velocitycomponent in the first direction and is introduced substantially evenlyaround the circumference of the container.
 2. A method as recited inclaim 1 wherein step (d) is practiced so that the first direction, inwhich the suspension flows, is vertically downwardly.
 3. A method asrecited in claim 1 wherein the suspension comprises sulphate pulp havinga consistency of about 8-15 percent.
 4. A method as recited in claim 3wherein the temperature of the sulphate pulp at step (a) is about80°-85° C., and wherein the treatment liquid is wash liquid and whereinthe temperature thereof at introduction is about 105°-110° C.